Unfortunately my mom has it as well, without the shoes on in the house as a trigger/assist. But she did work on her feet in shoes, so probably same deal ultimately. It was apparently exacerbated by something being off about her stride. I think she puts too much pressure on the inward side of her feet, or something like that.

Every 5 years or so she has to go in and get it cleaned up, it sounds incredibly painful. I think she had the fungus before she worked too though, it runs in her family. So far none of my siblings got it though. But my sisters have the same stride problem, so hopefully they can otherwise stave it off.